One of the methods to realize large-capacity optical communication is to increase the optical transmission speed.
However, it is known that simple increase in the optical transmission speed result in generation of distortion in a waveform which distortion is caused by wavelength dispersion of the optical transmission path (e.g., an optical fiber). This restricts the optical transmission.
The speed of light propagating through an optical transmission path varies with the wavelength (frequency) of the light, and such a variation widens the pulse of light, which is called the effect of wavelength dispersion of an optical transmission path. When an optical transmission path has wavelength dispersion, the presence of a number of frequency components in a light pulse, which frequency components are different in transmission speed, causes transmission light to have distortion in a waveform depending of the dispersion amount (the product of dispersion and distance).
When such distortion in a waveform is not compensated for, the amount of dispersion tolerant in optical transmission has a limit value, which is in inverse proportion ratio to approximate square of transmission speed of a signal. That means the tolerant limit becomes smaller in accordance with increase in transmission speed. In other words, a higher transmission speed of a signal makes the optical transmission less possible.
For example, in the cases where the bit rate of a transmission signal is about 2.5 Gbps, the light pulse has a relatively narrow spectrum width and therefore the pulse width is small, which has a less possibility in generation of inter symbol interference unless ultra long-haul transmission.
However, in the cases where the bit rate (speed) of a transmission signal is 10 Gbps or 40 Gbps, the spectrum width of the light pulse is wide as compared with that of a transmission signal of 2.5 Gbps and therefore the same amount of dispersion results in a larger pulse width. In the above cases, the narrow distance between light pulses increases the possibility of the generation of a small pulse spread to generate inter symbol interference.
For the above, it is preferable that wavelength dispersion caused by an optical transmission line is compensated for in accordance with increase of the bit rate of a transmission signal to 2.5 Gbps, 10 Gbps, 40 Gbps, and 100 Gbps.
There have been known the techniques of the non-patent references 1 through 4 as exiting techniques of dispersion compensation.
For example, one of the known methods converts a modulated complex optical electric field into a digital signal and compensates for distortion caused by wavelength dispersion of signal light through digital complex-filter processing (see Non-patent reference 1 below).
In order to reduce the circuit scale of a distortion compensating circuit, another method teaches that signal light is converted from a time domain to a frequency domain and wavelength dispersion is compensated for in a frequency domain (see Non-Patent reference 2 below).
Further, there have been known a method of monitoring an amount of wavelength dispersion (see Non-Patent reference 3 below) and a method of compensating for non-linear distortion (see Non-Patent Reference 4).
Non-Patent Reference 1: G. Charlet et al., “Transmission of 81 channels at 40 Gbit/s over a Transpacific-Distance Erbium-only Link, using PDM-BPSK Modulation, Coherent Detection, and a new large effective area fibre.” ECOC 2008, Brussels, Belgium, Sep. 21-25, 2008, TH.3.E.3, Vol. 7, p. 29-30
Non-Patent Reference 2: B. Spinnler et al., “Adaptive Equalizer Complexity in Coherent Optical Receivers┘, ECOC 2008, Brussels, Belgium, Sep. 21-25, 2008, We.2.E.4, Vol. 3, p. 127-128
Non-Patent Reference 3: F. N. Hauske et al., “Optical Performance Monitoring from FIR Filter Coefficients in Coherent Receivers”, OFC/NFOEC 2008
Non-Patent Reference 4: Kazuo Kikuchi, “Electronic Post-compensation for Nonlinear Phase Fluctuations in a 1000-km 20-Gbit/s Optical Quadrature Phase-Shift Keying Transmission System Using the Digital Coherent Receiver” OPTICS EXPRESS, Jan. 21, 2008, Vol. 16, No. 2, p. 889-896
However, compensation for the above wavelength dispersion requires a long filter corresponding to a light signal received by the dispersion compensating apparatus. As a consequence, the hardware scale of the distortion compensating apparatus becomes large.